14-3-3 proteins regulate a diverse array of cellular client proteins by forming binary complexes (see, e.g., Fu, H. et al., Ann. Rev. Pharmacol. Toxicol. 2000, 40, 617-647). Some of these protein-protein interactions (PPIs) may play a direct role in the pathobiology of multiple types of cancer and neurological disorders, such as Alzheimer's disease and Parkinson's disease. 14-3-3 PPIs also may be involved in the pathobiology of diabetes and inflammation, and they may mediate the virulence of certain pathogenic bacteria and viruses. For at least these reasons, there is an interest in modulating 14-3-3 PPIs for therapeutic purposes.
Fusicoccin A and cotylenin A may offer an entry point to 14-3-3 PPI modulation. In plant biology, these phytotoxins are believed to target a preformed binary complex between 14-3-3 and plasma membrane H+-ATPase (PMA2). Binding of fusicoccin A at the rim of this PPI interface may prolong the lifetime of the 14-3-3•PMA2 complex by 90-fold. This stabilizing effect may result in irreversible activation of PMA2, which, in turn, may stimulate rapid acidification of the cell wall.
Fusicoccin A and cotylenin A also may be active in human cell culture. Both molecules may sensitize cancer cells to apoptosis in combination with INF-α. The same regimen of fusicoccin A and cotylenin A and INF-α can be non-toxic to healthy cells. This targeted pro-apoptotic activity, while valuable, may reflect a complex pharmacology. Multiple cellular targets of fusicoccin A and cotylenin A have been identified, including a growing list of human 14-3-3 PPIs. This group includes estrogen receptor-α (ERα), which may be a master regulator of tumor proliferation expressed in the majority of breast cancers. Fusicoccin A has been identified as a possible stabilizer of a regulatory 14-3-3•ERα complex. Exposure of MCF-7 breast cancer cells to fusicoccin A (1.0 μM) may reduce or inhibit estradiol-stimulated ERα dimerization, downstream gene expression, and/or deceased cell proliferation. Thus, the anti-estrogenic activity of fusicoccin A may represent a new treatment approach for breast cancer types that are resistant to aromatase inhibitors and/or antiestrogen hormone therapy.
Although the non-optimized pharmacology of fusicoccin A and cotylenin A may directly impact cancer, the structural complexity of fusicoccin A, cotylenin A, and related fusicoccanes has left isolation from fungi as the only known way to access these molecules and their derivatives. The stabilizing properties of fusicoccin A and cotylenin A typically require the formation of multiple contact points with both proteins in 14-3-3•CP complexes.
Fusicoccadiene synthease has been identified, which is the terpene cyclase responsible for assembling the 5-8-5 carbotricycle of fusicoccin A from geranylgeranyl pyrophosphate. Dioxygenases that mediate key C—H oxidations in the biosynthesis of fusicoccin A also have been identified. These discoveries have seeded efforts to engineer biosynthetic entry to fusicoccanes.
By relying on strategy-level imitation of terpene biosynthesis, a fully synthetic route to di- and sesterterpenes harboring 5-8-5 carbotricycles has been developed. This approach may be valuable for preparing the ophiobolins from simple polyprenyl-derived starting materials. Access to the fusicoccane motif also has been reported; however, the direct application of this synthetic blueprint to the higher oxidation-state fusicoccin subfamily has not been demonstrated.
Other generalized approaches to 5-8-5 ring systems have been attempted. These include strategies that use UV-light to initiate cycloaddition reactions between tethered chromophores of notable complexity. It has been shown that diene tethers may provide entry to the 5-8-5 tricycle via metal-catalyzed [4+4] cycloaddition. The utility of a ring-expanding oxy-Cope strategy to prepare the fusicoccane nucleus also has been tested. These techniques, however, typically require lengthy sequences to assemble cyclization precursors, which impedes practical access to analogs and/or in-depth structural studies.
A complementary approach to fusicoccane variants has been developed that relies on semi-synthesis, wherein biosynthetic fusicoccin A and cotylenin A are prepared by fermentation, isolated, and then modified using chemical synthesis.
There remains a need for methods that overcome one or more of the foregoing disadvantages, including methods of synthesis that permit relatively easy access to the shared 5-8-5 core of fusicoccane derivatives. There also remains a need for fusicoccane derivatives, which may permit the selectivity for 14-3-3 PPI interfaces to be tailored.